Stents have been known and in use for several decades. Some early types of stent were formed from wire of stainless steel or, more recently, of Nitinol or other shape memory material. The wire is formed into a series of rings, in which typically the wire of each ring has a curved or substantially zigzag shape to enable this to be compressed onto an introducer and to give the ring an adjustable radial dimension for a good fit into the lumen into which the stent is in use positioned. The rings are connected together to form a unitary structure by sets of tie bars. In such early types of stents, the coupling of the tie bars to the wire rings has been by any suitable bonding such as welding.
While such a stent structure can perform satisfactorily, the stent has reduced compressibility and expansion characteristics which, moreover, cannot be finely predicted or adjusted. This can lead to their unsuitability in some more delicate applications, such as in smaller and more delicate vessels including, for instance, cerebral vessels.
More recently, there have been provided stents which are formed from a tube of raw material in which the stent structure is laser cut from the tube. The Applicant's Zilver® Stent is an example of such a laser cut stent. The advantage of such a stent structure is that it has greater uniformity in its characteristics, greater compressibility and much greater predictability in its performance. For instance, the individual stent rings can compress much tighter than an equivalent ring formed from wire, as adjacent struts of the stent ring are formed by cutting rather than bending of a wire. Moreover, the stent structure can be designed to have accurate and reproducible flexibility and also to have a varying flexibility as desired, for instance by cutting the stent rings or the struts forming these to different or varying widths or thicknesses.
Whilst laser cut stents can have significantly superior performance characteristics, they are expensive to manufacture particularly when produced from expensive materials such as Nitinol, in that the procedure is time consuming and there is a large wastage of material. Moreover, the laser cutting operation is complex particularly given that it generally needs to be adaptable so as to be able to produce different stents, with different dimensions and characteristics in dependence upon the specific medical application to which they are to be used. In addition to these difficulties, should there be any defects in the stent, such as a defect occurring during manufacture or during subsequent handling of the stent, the entire stent must be discarded.
There are known various types of modular stent assemblies, disclosed for example in WO-2004/110312, U.S.-2002/0111671, WO-00/15151 and U.S.-2006/0173529. Generally, these modular assemblies are intended to give the stent structure different characteristics in different operating conditions, such as to be radially flexible when compressed onto an introducer and then to become radially stiffer when expanded and in situ in a patient.